What is a vCPU on a C7a instance

Short answer: A vCPU on a C7a instance maps to a physical core. On other x86-based systems, a vCPU maps to a "virtual CPU" or "CPU thread," which represents one-half of a physical core. All of these instance types support symmetric multi-processing (SMP), which allows the OS to map application threads to different vCPUs.

More details:

Symmetric multi-threading (SMT), also known as "Hyper-Threading" with Intel processors, divides a physical CPU core into two virtual cores to improve overall performance and scalability. To the operating system and applications, these virtual cores appear to be real CPUs and are used as such, but in fact, they are not real CPUs. These two virtual cores can sometimes perform better together than a single physical core does, but they do not have the performance of two physical cores. SMT is not enabled on AWS Graviton-powered instances or C7a instances using 4th-generation AMD EPYC (Genoa) processors.

By way of example, a C6a or C6i instance having 16 vCPUs will use 8 physical cores. But a C7a instance that has 16 vCPUs will use 16 physical cores. This means you will get the full power of each physical core on the C7a instance with each vCPU.